Large plastic tanks, such as tanks of about 350 to 1250 gallons or more in nominal volume, have been widely used as septic tanks for treating wastewater, and also for storing water and other liquids. Typically, such large tanks are made by one of three processes: (1) laying up fiberglass fibers or mats in molds and bonding them together with a thermoset resin, such as polyester resin; (2) blow molding, in which a parison or mass of molten thermoplastic is positioned within a mold and forced by gas to expand outwardly against the walls of the mold cavity; and (3) rotational molding, in which a heated mold is rotated simultaneously on two or three axes while powdered thermoplastic is deposited into the mold, so that the plastic melts and adheres to the mold walls.
It is common for plastic septic tanks to have generally rectangular or round cross sections and to be buried so the length is horizontal. Such tanks require access openings at the top. Typically there are two spaced-apart circular openings of 1-2 feet in diameter, which are fitted with removable lids. The access openings permit placement of fittings and baffles within the tank at the time of installation. They also enable maintenance of the tank during use. For instance, exit end filters need to be cleaned and accumulated undigestible debris (commonly called sludge) must be periodically removed from the bottom of the tank.
The tanks must resist stresses which result from handling, storage and installation, in particular during backfilling of the hole in the earth into which a tank is placed. During use, tanks must resist the weight of overlying soil and possible vehicles crossing the soil surface, as well as the inward pressure of the soil and surrounding water, particularly when the tank is emptied for maintenance.
It is always an aim to minimize the weight of material needed to make a tank. The relationship between length and cross section is mostly such as to efficiently use plastic material. And the tanks, which commonly have walls of one-quarter to three-eights of an inch in thickness, are typically heavily corrugated to increase section modulus. Still, there is some tendency for thermoplastic tanks to deform or distort in vicinity of the top access openings, to the point that the lids or risers which attach to the tank will not properly fit the tank opening, either upon initial installation of the tank, or when covers are reinstalled after maintenance. Despite the past efforts of designers, the top of a molded tank may distort to the extent that a typical circular access opening becomes oblong (usually because of inward thrust of the lengthwise walls), or because the desired flat upper surface of the tank at the opening becomes twisted. When that occurs, the lid or other component mounted on the flange can have a poor fit, and in a worse case, soil and the like can fall into the tank. There also may be a failure to meet regulatory requirements related to joint leakage.
In fiberglass tanks, localized distortion of a tank can be countered by design changes involving preferential fiber orientation, or extra plies of fiberglass mat material. However, in the blow molded and rotationally molded thermoplastic tanks, the material properties of the typical HDPE polyethylene or other polyolefin material are substantially the same in all directions and it is not possible to vary them selectively at high stress locations. And neither blow molding nor rotational molding readily admit achieving localized regions of greatly increased thickness. Welding or otherwise affixing structure inside or outside the tank, to help the tank resist distortion, can be costly. Thus, there is a need for improved construction of tanks which enables provides better resistance to deformation in vicinity of access openings.